The present invention relates to a power supply for an electrostatic filter.
To scrub waste gas or more generally, to separate foreign matter from a flowing medium, electrostatic filters are frequently used to whose plates and spray wires a d-c voltage of such magnitude is applied that in the medium conducted between the plates and the spray wires there occurs an ionization of the foreign matter contained in it and the foreign matter precipitates on the plates. In the interest of a high precipitation rate the d-c voltage (supply voltage) of the plates and spray wires is selected as high as possible. On the other hand, at a high supply voltage, ionization processes also take place in the gas itself, leading to a constant filter discharge, up to a corona discharge at the spray wires.
If the supply voltage increases beyond a limit value, the filter will discharge during short breakdowns or even during voltage breakthroughs, up to a stationary arc, unless the direct current furnished by the supply voltage is interrupted. Up to the subsequent re-establishment of a high d-c voltage, no noteworthy precipitation of foreign matter is then possible. In addition, these processes cause filter wear, particularly of its spray wires, and a short service life of the entire device.
The ionization processes and, hence, the mentioned supply voltage limit, depend on the electric field strength distribution between the plates of the electrostatic filter. Insulating layers of foreign matter deposited on the plates must be knocked off, collected and removed in certain time intervals--possibly while shunting off the supply voltage as briefly as possible. Furthermore, space charges with severe distortions of the potential difference between the plates will form due to the ionization, it being even possible for a reversal of the voltage gradient and spray direction to occur between plates and space charges.
Thus, the mentioned limit value is not constant during operation. For good precipitation, the filter supply voltage should be kept as closely as possible at this limit value, which virtually changes uncontrollably.
Commercially available electrostatic filters contain a power supply connected to two phases of a three-phase supply line and drawing from the supply line an alternating current via an electronic chopper. The output voltage of the chopper is phase-angle controlled via the firing angle and furnishes an alternating current of supply frequency which is phase-shifted relative to the input voltage and which, after step-up and rectification, then feeds the electrostatic filter as pulsating, continuous current. To come close to the optimum working conditions of the filter, DE-AS No. 19 23 952 suggests to increase the voltage at the electrostatic filter through the phase-angle control in the chopper according to a certain step-up function until the limit value corresponding to the momentary filter state is reached and a voltage breakdown or a similar sudden discharge of the filter takes place.
After a breakdown, the a-c chopper must usually be blocked first to avoid an arc and to wait for the deionization of the plasma formed. The no-current minimum pause is determined by the chopper frequency, i.e. the supply line frequency. It follows therefrom that the filter is fed by a direct current flowing virtually without a gap having a ripple corresponding to the supply line frequency and interrupted after a breakdown. The resultant curve of the filter voltage fed by this current is wavy and rises up to the breakdown.
Electrostatic filters have already been suggested in which it has been omitted to supply the filter with such a virtually gapless flowing direct current drawn from the supply line by an a-c chopper of supply line frequency, stepped up and rectified. Rather, the filter is charged by a sequence of individual voltage or d-c pulses. To replenish with each pulse the charge which has flowed across the medium during the interpulse periods, the frequency and/or the duration of the individual pulses are specified so that the mean current density of these isolated d-c pulses assumes a filter set current value matched to the respective filter state. This causes a filter voltage to be produced which has a ripple according to the pulse repetition frequency and is below the breakdown limit, if possible.
This causes the technical difficulty of making the required energy available to the filter by means of the short pulses. U.S. Pat. No. 3,641,740 suggests in this regard to charge, by means of the rectified supply line voltage, a series of capacitors which are then connected to the electrostatic filter via thyristors, high-voltage transformers and a halfwave rectifier. The width of the current pulses reaching the electrostatic filter is, e.g., 5% of the interpulse period between these pulses.
Today, a combination is sought as the optimum method in which the filter is first biased by a rectifier with an already relatively high, virtually constant, basic d-c voltage to which are then superposed an alternating voltage or isolated, individual voltage pulses for the generation of a wavy filter voltage.
According to U.S. Pat. No. 3,984,215, their level should be considerably above the breakdown voltage of the filter, but should be obtained through a very short pulse duration so that no arc will form when the filter discharges. Duration, shape and pulse repetition frequency of these isolated, individual pulses are matched to the respective loading condition of the filter. According to European Patent No. 0 034 075, there are fed to the filter, biased to the constant, d-c base voltage, isolated current pulses whose maximum amplitude is controlled in accordance with a set filter current value so that the filter is thereby charged in the form of pulses to a maximum voltage below the breakdown voltage. These current pulses are taken from a rectifier-fed intermediate circuit by means of a resonant-circuit converter designed for the desired pulse width or by means of an automatic frequency-controlled frequency changer with current stepping up. The filter voltage ripple is also assured in that a diode suppresses one polarity of the stepped up current pulses.
DE-OS No. 27 13 675 suggests a simple power supply in which the base voltage is furnished by a phase angle-controlled a-c chopper connected to two phases of a three-phase supply line succeeded by a transformer and rectifier. The electrodes, fed by the d-c base voltage, are connected via a coupling capacitor to the secondary winding of a high-voltage transformer whose primary winding is fed by a controlled rectifier via a Y-point tapped inverter. Thus, an unrectified alternating voltage of a frequency variable between 50 Hz and 2 kHz as a function of the load is superposed to the base voltage.
If these methods, determined by the characteristics of the precipitation process, are to be applied at the operating site of the filter, the requirements to be met by the supply network must also be taken into consideration, as they are becoming stricter and stricter. For instance, the reactive current and harmonics loading of the supply network as well as an asymmetrical load between the three-phase terminals of the supply line network must be taken into account. Finally, the installation costs should be kept as low as possible.